3.3.1.1 Inserting letter to which you're replying

To control how much of the message to which you are replying is yanked
by C-c C-y (mh-yank-cur-msg) into your reply, modify
mh-yank-from-start-of-msg. The default value of t means
that the entire message is copied. If it is set to 'body (don't
forget the apostrophe), then only the message body is copied. If it is
set to nil, only the part of the message following point (the
current cursor position in the message's buffer) is copied. In any
case, this variable is ignored if a region is set in the message you are
replying to. The string contained in mh-ins-buf-prefix is
inserted before each line of a message that is inserted into a draft
with C-c C-y (mh-yank-cur-msg). I suggest that you not
modify this variable. The default value of `"> "' is the default
string for many mailers and news readers: messages are far easier to
read if several included messages have all been indented by the same
string. The variable mail-citation-hook is nil by
default, which means that when a message is inserted into the letter,
each line is prefixed by mh-ins-buf-prefix. Otherwise, it can be
set to a function that modifies an included
citation.
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If you like to yank all the text from the message you're replying to in
one go, set mh-delete-yanked-msg-window to non-nil to delete
the window containing the original message after yanking it to make more
room on your screen for your reply.